1. The Field of the Disclosure
The present disclosure relates generally to airfoils or watercraft structures, and more particularly, but not necessarily entirely, to airfoils utilizing a twist distribution to optimize lift.
2. Description of Related Art
Some important differences between the concepts of induced drag and lift are set forth below.
A. Induced Drag
Induced drag is caused by the generation of lift by a wing and is parallel to the relative wind into which the wing is flying. When a wing flies at the zero lift angle of attack there is no lift and therefore no induced drag. Conversely, when the angle of attack increases the wing produces more lift, therefore there is more induced drag. The magnitude of the induced drag depends on (1) the amount of lift being generated by the wing; and (2) on the shape and size of the wing, also known as wing planform. As might be expected, induced drag is undesirable while flying in that it results in diminished fuel economy as well as decreased airspeed. Induced drag also contributes to the stall characteristics of a given wing.
The prior art teaches various features that may be incorporated into a wing in order to reduce induced drag at high angles of attack. One of the more well known ways to reduce induced drag is to increase the wingspan. For example, this would include aircraft such as gliders, as well as high altitude spy planes such as the U2. It also includes to a lesser degree modern jet airliners. However, as the span is increased, the wing structural weight also increases and at some point the weight increase offsets the induced drag savings.
Another previously known method for reducing induced drag is to employ end plates onto the tips of the wings. The end plates served to block some of the vortices causing reduced drag. However, end plates are not employed widely due to their relative inefficiencies. Still another method for reducing drag is using winglets. Other known attempts to reduce induced drag include wings with slotted edges and wings with fanned partial wings.
Tapered wings are also commonly used as a means for reducing induced drag. It can be shown that tapered wings with the right amount of taper have a lower reduced drag than an untapered wing. However, this reduction comes at a price. A tapered wing tends to stall first at in the region near the wingtips. This wingtip stall can lead to poor handling characteristics during stall recovery. Thus, tapered wings have commonly been used as a compromise solution.
Around the 1920s it was found that a flat elliptical shaped wing gave a uniform air deflection along the entire span, which minimized the induced drag. Elliptical shaped wings were used on the British SuperMarine Spitfire, a popular WWII fighter, to reduce induced drag. In fact, it can be shown that an elliptical wing produces the minimum possible induced drag for all angles of attack. Unfortunately, there are several problems with elliptical wings. First, elliptical shaped wings are cost prohibitive. While this barrier is less important today than it once was, provided that the designer is willing to use modern composite materials. However, making an elliptical shape out of aluminum is quite difficult and therefore expensive. Next, elliptical wings have undesirable stall characteristics. It is much safer to design an airplane so that the wing stalls first at the root, leaving the outer portion of the wing, (where the ailerons are) still flying. An elliptical wing however, will tend to stall uniformly all along the span creating a potentially dangerous situation for the pilot. Finally, other factors dictate a wings ideal shape more than the desire to reduce induced drag. The tapered wing, for instance, is lighter and easier to build, factors which outweigh the advantages of an elliptical wing's ability to reduce induced drag.
Another popular method of reducing induced drag is to design a wing with washout, also referred to herein as twist or wing twist. Washout may be applied to wings so that the outboard section of the wing does not stall first. When an aircraft may be increasing its angle of attack, i.e. increasing the lift of the wing, the airflow over the wing eventually reaches a point where it becomes separated, causing a loss in lift. By twisting the front outboard portion of the wing down, the lift and induced drag in that area may be decreased and the stall may be delayed in that area. By maintaining lift on the outboard portion of the wing, the pilot may be still able to maintain roll control of the aircraft in the event of a stall on other portions of the wing.
Conventionally, washout may be incorporated into a wing using geometric twist and aerodynamic twist. The use of washout in the prior art, however, may be characterized by two major shortcomings. First, since the amount of twist may be integrated into a wing at the time of construction, usually for a design lift coefficient, the twist in a wing may only be optimized, if at all, for one portion of the expected flight envelope. Second, washout comes at a price. A wing with washout experiences a decrease in lift performance due to the reduction in the angle of attack.
B. Lift
Lift is the force that is perpendicular to the direction of flight. For example, lift operates to oppose the weight of an aircraft and hold the aircraft in the air as well as other functions, such as turning and maneuvering the aircraft. Lift is a mechanical aerodynamic force produced by the motion of the airplane through the air. Lift is generated by every part of the aircraft, but most of the lift on a conventional aircraft is generated by the wings.
There are several factors which affect the magnitude of lift. Lift depends on the density of the air, the square of the velocity, the air's viscosity and compressibility, the surface area over which the air flows, the shape of the body, and the body's inclination to the flow. In general, the dependence on body shape, inclination, air viscosity, and compressibility is very complex and difficult to calculate. One way to deal with these complex dependencies is to characterize the dependence by a single variable. For lift, this variable is called the lift coefficient. This allows aircraft designers to collect all the effects, simple and complex, into the single lift equation below:
  L  =                    C        L            ⨯              S        w            ⨯              V        2            ⨯      ρ        2  In the above equation, CL is the coefficient of lift, ρ is the density of air (air density is calculated here as a function of temperature and pressure), V is the velocity or airspeed, SW is the surface area of the lifting surface, and L is the lift force produced. For given air conditions, shape, and inclination of the object, one has to determine a value for CL to determine the lift. For some simple flow conditions and geometries and low inclinations, aerodynamicists can determine the value of CL mathematically. But, in general, the lift coefficient parameter, CL, is determined experimentally using wind tunnels. In addition, various values of CL for different air foil sections are published in reference tables. In short, it can be understood that the lift coefficient is a parameter associated with a particular shape of an airfoil, and is incorporated in the lift equation to predict the lift force generated by a wing using this particular cross section.
During takeoff and landing of an aircraft, it is desirable that the lift of the aircraft is maximized while maintaining as low as possible velocity for safety reasons. In the past, one way to increase lift while keeping a relatively low velocity was to deploy wing flaps. Wing flaps are a movable part of the wing, normally hinged to the trailing edge (rear edge) of each wing closest to the airplane body. The pilot extends and retracts the flaps. Extending the flaps increases the wing camber and the angle of attack of the wing. This increases wing lift and also increases drag. Flaps enable the pilot to make a steeper descent when landing without increasing airspeed. They also help the airplane get off the ground in a short distance. There are many different types of flaps. Some hinge, some slide, some open with slots, and some help smooth the air over the wing even when high angles of attack are flown during landing.
In addition to flaps, slats may be used to increase lift. Slats are protrusions from the leading edge (front edge) of a wing. They add to the lift of a wing. Slats and flaps work together to maintain laminar flow (a smooth airflow) over the top of the wing. When cruising at a desired altitude, flaps and slats are retracted to minimize drag. While flaps and slats have been effectively employed in the past, flaps and slats do not necessarily obtain the maximum total possible lift for a wing of a given design.
The prior art is thus characterized by several disadvantages that are addressed by the present disclosure. The present disclosure minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.
The features and advantages of the disclosure will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the disclosure without undue experimentation. The features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.